1) Field of the Invention
The present invention relates to a substrate, more specifically to a substrate adapted for interconnecting optical elements, which is suitably applicable for carrying out optical coupling between optical elements, and furthermore specifically to a substrate adapted for interconnecting optical elements as well as optical module, which are suitably applicable for data transmission system.
2) Description of the Related Art
In the data transfer system today, for example, in a server system, while data processing is being carried out by use of an electric signal, data transfer proceeds by use of an optical signal. In the data transfer using the optical signal in such a server system, optical modules are in use. An optical module is an arrangement, which allows a photoelectric converter and an optical element for propagating the optical signal, e.g., an optical fiber to be connected and optically coupled.
Specifically, by connecting a photoelectric converter, which converts the electric signal into the optical signal, and an optical fiber, there can be configured a transmission module, which transmits the optical signal converted by the photoelectric converter through the optical fiber, while by connecting the photoelectric converter, which converts the optical signal into the electric signal, and the optical fiber, there can be configured a reception module, which converts the optical signal transmitted through the optical fiber into the electric signal by the photoelectric converter.
In the advanced server system today, the parallel transfer of an optical signal on a large scale is based upon the premise that multiple optical modules are mounted in parallel, so that small-type, low manufacturing cost and low dissipation power are demanded of each optical module to be applied to such a system.
In order to develop a small, low-cost optical module, it is important to reduce the number of components constituting the module, and in order to develop a low-dissipation power optical module, it is important to achieve an efficient and stable optical coupling in both transmission modules and reception modules, so as to minimize optical transmission power to the extent of communicability.
In the optical module for communications developed so far, in view of the need for long-distance transmission the design has been carried out with consideration as to how efficiently the optical element and optical fiber are coupled, thus a number of lens couplings of high-cost and yet high-coupling-efficiency have been largely employed.
As for a technique to connect and secure the optical element using the above mentioned lens coupling, for example, optical module 100 of can package type, can be cited, which is shown in FIG. 23. The optical module 100 shown in FIG. 23 is provided with an optical element 101, two pieces of lenses 102 and 103, a ferrule 104, a cap with a window 105, a stem 106 and a flexible circuit board 107. That is to say, as components for connecting and securing the optical element 101 and ferrule 104 there are provided two pieces of lenses 102 and 103, a cap with a window 105, a stem 106 and a flexible circuit board 107.
Another example is described in the Patent Document 1 described below. In the patent document 1, as shown in FIG. 24, a description is given about an optical sub-assembly 110, which is provided with a flexible circuit 116, a photoelectric element 118, a spacer 114, a micro-lens eye 136 and an optical member 112 for supporting the micro-lens eye 136, wherein the reduction of manufacturing cost is sought by employing molded components. Additionally, in FIG. 24, as components for connecting and securing the photoelectric converter 118 and an optical fiber block 123, the flexible circuit 116, spacer 114, micro-lens eye 136 and optical member 112 are provided.
Other well known techniques in the art related to the present invention are, for example, those described in Patent Documents 2 to 5 as follows:    (Patent Document 1) Japanese Patent Laid-Open (Kokai) No. 2000-82830    (Patent Document 2) Japanese Patent Laid-Open (Kokai) No. HEI 09-270747    (Patent Document 3) Japanese Patent Laid-Open (Kokai) No. HEI 11-233911    (Patent Document 4) Japanese Patent Laid-Open (Kokai) No. 2002-98842    (Patent Document 5) Japanese Patent Laid-Open (Kokai) No. HEI 04-329699
However, in the optical module, which is shown in the above mentioned FIG. 23, since an optical element 101 and a ferrule 104 are optically coupled by lenses 102 and 103, they have a relatively favorable optical coupling efficiency, though, there is a problem in promoting miniaturization and cost reduction because the number of components are comparatively large.
Additionally, in the technique described in Patent Document 1, since the number of components is relatively large, there is the same problem in the case of FIG. 23, and additionally in order to improve the coupling efficiency needed for lower power consumption, another difficulty arises as follows.
That is, the coupling efficiency between the micro-lens and the optical fiber becomes relatively lower than that of the one shown in FIG. 23, especially between the light emitting element and the single-mode fiber. Furthermore, since a spacer 114 is configured by resin molding and has a relatively larger size, a thickness error tends to occur. Therefore, errors also tend to occur regarding the interval between the optical element and the micro-lens, in some cases causing a problem to stabilize the optical coupling efficiency.
In addition, the technique described in the above mentioned Patent Document 2 to 5 is not intended to provide a technique for promoting miniaturization and cost reduction through the above mentioned reduction of the number of components and concurrently for improving the coupling efficiency needed for low power consumption.